US20120307404A1 - Three-terminal spin-torque oscillator (sto) - Google Patents
Three-terminal spin-torque oscillator (sto) Download PDFInfo
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- US20120307404A1 US20120307404A1 US13/149,419 US201113149419A US2012307404A1 US 20120307404 A1 US20120307404 A1 US 20120307404A1 US 201113149419 A US201113149419 A US 201113149419A US 2012307404 A1 US2012307404 A1 US 2012307404A1
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- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F10/00—Thin magnetic films, e.g. of one-domain structure
- H01F10/32—Spin-exchange-coupled multilayers, e.g. nanostructured superlattices
- H01F10/324—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer
- H01F10/3254—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer the spacer being semiconducting or insulating, e.g. for spin tunnel junction [STJ]
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- G—PHYSICS
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/02—Measuring direction or magnitude of magnetic fields or magnetic flux
- G01R33/06—Measuring direction or magnitude of magnetic fields or magnetic flux using galvano-magnetic devices
- G01R33/09—Magnetoresistive devices
- G01R33/098—Magnetoresistive devices comprising tunnel junctions, e.g. tunnel magnetoresistance sensors
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
- G01R33/00—Arrangements or instruments for measuring magnetic variables
- G01R33/12—Measuring magnetic properties of articles or specimens of solids or fluids
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/127—Structure or manufacture of heads, e.g. inductive
- G11B5/33—Structure or manufacture of flux-sensitive heads, i.e. for reproduction only; Combination of such heads with means for recording or erasing only
- G11B5/39—Structure or manufacture of flux-sensitive heads, i.e. for reproduction only; Combination of such heads with means for recording or erasing only using magneto-resistive devices or effects
- G11B5/3903—Structure or manufacture of flux-sensitive heads, i.e. for reproduction only; Combination of such heads with means for recording or erasing only using magneto-resistive devices or effects using magnetic thin film layers or their effects, the films being part of integrated structures
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F10/00—Thin magnetic films, e.g. of one-domain structure
- H01F10/32—Spin-exchange-coupled multilayers, e.g. nanostructured superlattices
- H01F10/324—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer
- H01F10/3286—Spin-exchange coupled multilayers having at least one layer with perpendicular magnetic anisotropy
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F10/00—Thin magnetic films, e.g. of one-domain structure
- H01F10/32—Spin-exchange-coupled multilayers, e.g. nanostructured superlattices
- H01F10/324—Exchange coupling of magnetic film pairs via a very thin non-magnetic spacer, e.g. by exchange with conduction electrons of the spacer
- H01F10/329—Spin-exchange coupled multilayers wherein the magnetisation of the free layer is switched by a spin-polarised current, e.g. spin torque effect
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N50/00—Galvanomagnetic devices
- H10N50/10—Magnetoresistive devices
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
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- H—ELECTRICITY
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Definitions
- the invention relates generally to a spin-torque oscillator (STO), and more particularly to a magnetic field sensor and sensing system that uses a STO sensor.
- STO spin-torque oscillator
- a GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu).
- a nonmagnetic electrically conductive spacer layer which is typically copper (Cu).
- Cu copper
- One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer.
- the other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer.
- the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field, such as from the recorded magnetic bits on the disk, is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
- CPP current-perpendicular-to-the-plane
- CPP-GMR read heads In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer.
- the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers.
- the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag.
- the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO 2 , MgO or Al 2 O 3 .
- CPP MR sensors In CPP MR sensors, it is desirable to operate the sensors at a high bias or sense current density to maximize the signal and signal-to-noise ratio (SNR).
- SNR signal and signal-to-noise ratio
- CPP MR sensors are susceptible to current-induced noise and instability.
- the spin-polarized bias current flows perpendicularly through the ferromagnetic layers and produces a spin-torque (ST) effect on the local magnetization. This can produce fluctuations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is large.
- STO spin-torque oscillator
- STO sensors have been proposed for use as read heads in magnetic recording disk drives to replace conventional CPP-GMR and CPP-TMR read heads, as described for example in US 20100328799 A1 assigned to the same assignee as this application, and in US 20090201614 A1.
- An STO sensor based on a CPP-GMR sensor can operate at very high current densities due to its nonmagnetic conductive spacer layer between the reference and free layers, but has a very small output signal as a result of its low magnetoresistance (AR/R).
- An STO sensor based on a CPP-TMR sensor has a significantly higher magnetoresistance but is susceptible to dielectric breakdown of the tunnel barrier at high current density.
- a STO for use in a system, like a magnetic field sensing system, that has a high output signal that is not susceptible to dielectric breakdown of the tunnel barrier.
- the invention is a spin-torque oscillator (STO) with a single free layer that forms part of both a GMR structure with a nonmagnetic conductive spacer layer and a TMR structure with a tunnel barrier layer.
- the STO has three electrical terminals that connect to electrical circuitry that provides a spin-torque excitation current through the conductive spacer layer and a lesser sense current through the tunnel barrier layer.
- the STO has applications for use as an oscillator in mixers, radios, cell phones and radar (including vehicle radar), and as an oscillator in microwave-assisted magnetic recording (MAMR).
- the STO is a magnetic field sensor, such as a current-perpendicular-to-the-plane (CPP) disk drive read head.
- the STO sensor has a single free ferromagnetic layer that has an in-plane magnetization substantially free to oscillate in the presence of external magnetic fields to be sensed, such as the magnetized “bits” or regions on the disk.
- the free layer forms part of both a TMR structure with tunnel barrier layer and a first reference layer having a fixed in-plane magnetization, and a GMR structure with a nonmagnetic conductive spacer layer and a second reference layer having a fixed in-plane magnetization.
- the STO sensor has three electrical contacts or terminals for connection to electrical circuitry.
- a first terminal is electrically coupled to the first reference layer, a second terminal is electrically coupled to the second reference layer, and a third terminal is electrically coupled to either the conductive spacer layer or the free layer.
- the electrical circuitry connected to the STO terminals includes an excitation current source and a sense current source.
- the excitation current is greater that the critical current for the GMR structure and is high enough to provide sufficient current density to cause the magnetization of the free layer to oscillate at a fixed base frequency in the absence of an external magnetic field.
- the sense current is less than the critical current for the TMR structure.
- a detector coupled to the sense current detects shifts in the free layer magnetization oscillation frequency from the base frequency in response to the external magnetic fields from the magnetized regions of the disk.
- FIG. 1 is a schematic top view of a conventional magnetic recording hard disk drive with the cover removed.
- FIG. 2 is an enlarged end view of the slider and a section of the disk taken in the direction 2 - 2 in FIG. 1 .
- FIG. 3 is a view in the direction 3 - 3 of FIG. 2 and shows the air-bearing surface (ABS) of the slider with the ends of the read/write head.
- ABS air-bearing surface
- FIG. 4 is a cross-sectional schematic view of the ABS of a current-perpendicular-to-the-plane magnetoresistive (CPP MR) read head showing the stack of layers located between the magnetic shield layers according to the prior art.
- CPP MR current-perpendicular-to-the-plane magnetoresistive
- FIG. 5 is a schematic of a magnetic field spin-torque oscillator (STO) sensing system according to an embodiment of the invention in a magnetic recording disk drive implementation.
- STO magnetic field spin-torque oscillator
- FIG. 6 is a schematic of the embodiment of FIG. 5 with the STO sensor in reflection mode but wherein the second reference layer is not in the stack but recessed from the ABS.
- FIG. 7 is a schematic of an embodiment with the STO sensor in transmission mode but wherein both the second reference layer and the nonmagnetic conductive spacer layer are not in the stack but recessed from the ABS.
- FIG. 8 is a schematic of an embodiment with the STO sensor in transmission mode but wherein both the first reference layer and the tunnel barrier layer are not in the stack but are recessed from the ABS.
- the three-terminal STO according to the invention has applications other than as a magnetic field sensor, but will be described in detail below as magnetic recording disk drive read head.
- FIGS. 1-4 illustrate a conventional CPP magnetoresistive (MR) magnetic field sensing sensor and system.
- FIG. 1 is a block diagram of a conventional magnetic recording hard disk drive.
- the disk drive includes a magnetic recording disk 12 and a rotary voice coil motor (VCM) actuator 14 supported on a disk drive housing or base 16 .
- the disk 12 has a center of rotation 13 and is rotated in direction 15 by a spindle motor (not shown) mounted to base 16 .
- the actuator 14 pivots about axis 17 and includes a rigid actuator arm 18 .
- a generally flexible suspension 20 includes a flexure element 23 and is attached to the end of arm 18 .
- a head carrier or air-bearing slider 22 is attached to the flexure 23 .
- a magnetic recording read/write head 24 is formed on the trailing surface 25 of slider 22 .
- the flexure 23 and suspension 20 enable the slider to “pitch” and “roll” on an air-bearing generated by the rotating disk 12 .
- FIG. 2 is an enlarged end view of the slider 22 and a section of the disk 12 taken in the direction 2 - 2 in FIG. 1 .
- the slider 22 is attached to flexure 23 and has an air-bearing surface (ABS) 27 facing the disk 12 and a trailing surface 25 generally perpendicular to the ABS.
- ABS 27 causes the airflow from the rotating disk 12 to generate a bearing of air that supports the slider 20 in very close proximity to or near contact with the surface of disk 12 .
- the read/write head 24 is formed on the trailing surface 25 and is connected to the disk drive read/write electronics by electrical connection to terminal pads 29 on the trailing surface 25 . As shown in the sectional view of FIG.
- the disk 12 is a patterned-media disk with discrete data tracks 50 spaced-apart in the cross-track direction, one of which is shown as being aligned with read/write head 24 .
- the discrete data tracks 50 have a track width TW in the cross-track direction and may be formed of continuous magnetizable material in the circumferential direction, in which case the patterned-media disk 12 is referred to as a discrete-track-media (DTM) disk.
- the data tracks 50 may contain discrete data islands spaced-apart along the tracks, in which case the patterned-media disk 12 is referred to as a bit-patterned-media (BPM) disk.
- BPM bit-patterned-media
- the disk 12 may also be a conventional continuous-media (CM) disk wherein the recording layer is not patterned, but is a continuous layer of recording material.
- CM continuous-media
- FIG. 3 is a view in the direction 3 - 3 of FIG. 2 and shows the ends of read/write head 24 as viewed from the disk 12 .
- the read/write head 24 is a series of thin films deposited and lithographically patterned on the trailing surface 25 of slider 22 .
- the write head includes a perpendicular magnetic write pole (WP) and may also include trailing and/or side shields (not shown).
- the CPP MR sensor or read head 100 is located between two magnetic shields S 1 and S 2 .
- the shields S 1 , S 2 are formed of magnetically permeable material, typically a NiFe alloy, and may also be electrically conductive so they can function as the electrical leads to the read head 100 .
- the shields function to shield the read head 100 from recorded data bits that are neighboring the data bit being read. Separate electrical leads may also be used, in which case the read head 100 is formed in contact with layers of electrically conducting lead material, such as tantalum, gold, or copper, that are in contact with the shields S 1 , S 2 .
- FIG. 3 is not to scale because of the difficulty in showing very small dimensions.
- each shield S 1 , S 2 is several microns thick in the along-the-track direction, as compared to the total thickness of the read head 100 in the along-the-track direction, which may be in the range of 20 to 40 nm.
- FIG. 4 is a view of the ABS showing the layers making up a CPP MR sensor structure as would be viewed from the disk.
- Sensor 100 is a CPP MR read head comprising a stack of layers formed between the two magnetic shield layers S 1 , S 2 .
- the sensor 100 has a front edge at the ABS and spaced-apart side edges 102 , 104 that define the track width (TW).
- the shields S 1 , S 2 are formed of electrically conductive material and thus may also function as electrical leads for the bias or sense current I s , which is directed generally perpendicularly through the layers in the sensor stack. Alternatively, separate electrical lead layers may be formed between the shields S 1 , S 2 and the sensor stack.
- the lower shield S 1 is typically polished by chemical-mechanical polishing (CMP) to provide a smooth substrate for the growth of the sensor stack.
- CMP chemical-mechanical polishing
- a seed layer 101 such as a thin Ru/NiFe bilayer, is deposited, typically by sputtering, below S 2 to facilitate the electroplating of the relatively thick S 2 .
- the sensor 100 layers include a reference ferromagnetic layer 120 having a fixed magnetic moment or magnetization direction 121 oriented transversely (into the page), a free ferromagnetic layer 110 having a magnetic moment or magnetization direction 111 that can rotate in the plane of layer 110 in response to transverse external magnetic fields from the disk 12 , and a nonmagnetic spacer layer 130 between the reference layer 120 and free layer 110 .
- the CPP MR sensor 100 may be a CPP GMR sensor, in which case the nonmagnetic spacer layer 130 would be formed of an electrically conducting material, typically a metal like Cu, Au or Ag.
- the CPP MR sensor 100 may be a CPP tunneling MR (CPP-TMR) sensor, in which case the nonmagnetic spacer layer 130 would be a tunnel barrier formed of an electrically insulating material, like TiO 2 , MgO or Al 2 O 3 .
- CPP-TMR CPP tunneling MR
- the pinned ferromagnetic layer in a CPP MR sensor may be a single pinned layer or an antiparallel (AP) pinned structure like that shown in FIG. 4 .
- An AP-pinned structure has first (AP 1 ) 122 and second (AP 2 ) 120 ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC) layer 123 with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel.
- the AP 2 layer 120 which is in contact with the nonmagnetic APC layer 120 on one side and the sensor's electrically nonmagnetic spacer layer 130 on the other side, is typically referred to as the reference layer.
- the AP 1 layer 122 which is typically in contact with an antiferromagnetic layer 124 or hard magnet pinning layer on one side and the nonmagnetic APC layer 123 on the other side, is typically referred to as the pinned layer.
- the AP-pinned structure minimizes the net magnetostatic coupling between the reference/pinned layers and the CPP MR free ferromagnetic layer.
- the AP-pinned structure is sometimes also called a “laminated” pinned layer or a synthetic antiferromagnet (SAF).
- the pinned layer in the CPP GMR sensor in FIG. 4 is a well-known AP-pinned structure with reference ferromagnetic layer 120 (AP 2 ) and a lower ferromagnetic layer 122 (AP 1 ) that are antiferromagnetically coupled across an AP coupling (APC) layer 123 .
- the APC layer 123 is typically Ru, Ir, Rh, Cr or alloys thereof.
- the AP 1 and AP 2 layers, as well as the free ferromagnetic layer 110 are typically formed of crystalline CoFe or NiFe alloys, or a multilayer of these materials, such as a CoFe/NiFe bilayer.
- the AP 1 and AP 2 ferromagnetic layers have their respective magnetization directions 127 , 121 oriented antiparallel.
- the AP 1 layer 122 may have its magnetization direction pinned by being exchange-coupled to an antiferromagnetic (AF) layer 124 as shown in FIG. 4 .
- the AF layer 124 is typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn, PdMn, PtPdMn or RhMn.
- the AP-pinned structure may be “self-pinned”.
- the AP 1 and AP 2 layer magnetization directions 127 , 121 are typically set generally perpendicular to the disk surface by magnetostriction and the residual stress that exists within the fabricated sensor. It is desirable that the AP 1 and AP 2 layers have similar moments. This assures that the net magnetic moment of the AP-pinned structure is small so that magnetostatic coupling to the free layer 110 is minimized and the effective pinning field of the AF layer 124 , which is approximately inversely proportional to the net moment of the AP-pinned structure, remains high.
- a seed layer 125 may be located between the lower shield layer S 1 and the AP-pinned structure. If AF layer 124 is used, the seed layer 125 enhances the growth of the AF layer 124 .
- the seed layer 125 is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru.
- a capping layer 112 is located between the free ferromagnetic layer 110 and the upper shield layer S 2 .
- the capping layer 112 provides corrosion protection and may be a single layer or multiple layers of different materials, such as Ru, Ta, Ti, or a Ru/Ta/Ru , Ru/Ti/Ru, or Cu/Ru/Ta trilayer.
- the magnetization direction 111 of free layer 110 will rotate while the magnetization direction 121 of reference layer 120 will remain fixed and not rotate.
- a sense current I S is applied from top shield S 2 perpendicularly through the sensor stack to bottom shield S 1 (or from S 1 to S 2 )
- the magnetic fields from the recorded data on the disk will cause rotation of the free-layer magnetization 111 relative to the reference-layer magnetization 121 , which is detectable as a change in electrical resistance.
- a ferromagnetic biasing layer 115 such as a CoPt or CoCrPt hard magnetic bias layer, is also typically formed outside of the sensor stack near the side edges 102 , 104 of the sensor 100 .
- the biasing layer 115 is electrically insulated from side edges 102 , 104 of sensor 100 by insulating layer 116 .
- An optional seed layer 114 such as a Cr alloy like CrMo or CrTi, may be deposited on the insulating layer 116 to facilitate the growth of the biasing layer 115 , particularly if the biasing layer is a CoPt or CoPtCr layer.
- a capping layer 118 such as layer of Cr, or a multilayer of Ta/Cr is deposited on top of the biasing layer 115 .
- the upper layer of capping layer 118 for example Cr, also serves the purpose as a chemical-mechanical-polishing (CMP) stop layer during fabrication of the sensor.
- the biasing layer 115 has a magnetization 117 generally parallel to the ABS and thus longitudinally biases the magnetization 111 of the free layer 110 . Thus in the absence of an external magnetic field its magnetization 117 is parallel to the magnetization 111 of the free layer 110 .
- the ferromagnetic biasing layer 115 may be a hard magnetic bias layer or a ferromagnetic layer that is exchange-coupled to an antiferromagnetic layer.
- a seed layer 101 such as a NiFe layer, for the shield layer S 2 may be located over the sensor 100 and capping layer 118 .
- the magnetization direction 111 of free layer 110 will rotate while the magnetization direction 121 of reference layer 120 will remain substantially fixed and not rotate.
- the rotation of the free-layer magnetization 111 relative to the reference-layer magnetization 121 results in a change in electrical resistance.
- the resistance change is detected as a voltage signal proportional to the strength of the magnetic signal fields from the recorded data on the disk.
- I s is greater than some critical current (I c ) the spin-torque (ST) effect can produce gyrations or fluctuations of the free layer magnetization, resulting in substantial low-frequency magnetic noise that reduces the sensor's signal-to-noise ratio (SNR) to an undesirable level.
- I c critical current
- SNR signal-to-noise ratio
- STO spin torque oscillator
- STO sensors have been proposed for use as read heads in magnetic recording disk drives to replace conventional CPP-GMR and CPP-TMR read heads, as described for example in US 20100328799 A1 assigned to the same assignee as this application, and in US 20090201614 A1.
- An STO sensor based on a CPP-GMR sensor can operate at very high current densities due to its conductive spacer layer between the reference and free layers, but has a very small output signal as a result of its low magnetoresistance ( ⁇ R).
- An STO sensor based on a CPP-TMR sensor has a significantly higher magnetoresistance but is susceptible to dielectric breakdown of the tunnel barrier at high current density.
- FIG. 5 is a schematic of a magnetic field sensing system using a STO sensor 200 according to an embodiment of the invention.
- the system is illustrated as a magnetic recording disk drive with STO sensor 200 with its ABS facing the disk 250 .
- the sensor 200 includes a set of individual layers and features of both a CPP-GMR sensor and a CPP-TMR sensor as previously-described with respect to CPP sensor 100 .
- the disk 250 has a substrate 252 and a recording layer 254 that serves as the magnetic recording medium with magnetized regions depicted by the arrows directed toward or away from the ABS.
- the recording layer 254 is depicted as a perpendicular magnetic recording medium with the regions magnetized perpendicularly to the plane of recording layer 254 , but alternatively it may be a longitudinal magnetic recording medium with the regions being magnetized in the plane of recording layer 254 .
- the STO sensor 200 has a first shield layer S 1 that may serve as a substrate for the deposition of the set of layers, a second shield layer S 2 , and a single free ferromagnetic layer 210 that has a substantially in-plane magnetization 211 free to oscillate in the presence of an external magnetic field to be sensed.
- the free layer 210 forms part of both a TMR structure with tunnel barrier layer 230 and a first reference layer 220 having a fixed in-plane magnetization 221 , and a GMR structure with nonmagnetic conductive spacer layer 270 and second reference layer 260 having a fixed in-plane magnetization 261 .
- Each of the reference layers 220 , 260 may be a single pinned layer or the AP 2 layer of an AP-pinned structure.
- a non-ferromagnetic conducting metal layer 225 is located between S 1 and first reference layer 220 for breaking any magnetic exchange interaction between S 1 and first reference layer 220 or other ferromagnetic layers in the sensor stack while permitting electrical conduction.
- a non-ferromagnetic conducting metal layer 265 is located between S 2 and second reference layer 260 .
- Typical materials for layers 225 , 265 are Cu, Ag, Ta and Ru.
- the order of the sensor layers in the stack could be reversed from what is shown in FIG. 5 , with second reference layer 260 being deposited first on layer 265 on S 1 , followed by conductive spacer layer 270 , free layer 210 , tunnel barrier layer 230 , first reference layer 220 , and layer 225 , with S 2 being located on layer 225 .
- the STO sensor 200 has three electrical contacts or terminals for connection to electrical circuitry 300 .
- Terminal 301 is electrically coupled to the first reference layer 220 via S 1
- terminal 302 is electrically coupled to the second reference layer 260 via S 2
- terminal 303 is electrically coupled to either the conductive spacer layer 270 or to the free layer 210 .
- terminal 303 is connected to conductive spacer layer 270 , but may be connected to either the conductive spacer layer 270 or the free layer 210 .
- the terminals 301 , 302 , 303 are depicted for ease of illustration in FIG. 5 as being directly connected to their respective layers, but would likely be located on the trailing surface of the slider, as depicted in FIG. 2 by terminal pads 29 on trailing surface 25 of slider 22 .
- the sensor includes insulating material 290 in the back region recessed from the ABS for electrically isolating S 1 , conductive spacer layer 270 and S 2 from one another.
- the circuitry connected to STO sensor 200 includes a constant current source 310 that supplies a direct current (DC) excitation current I e between terminals 302 , 303 through the conductive spacer layer 270 , and a constant current source 320 that supplies a direct DC sense current I s between terminals 301 , 303 through the tunnel barrier layer 230 .
- the excitation current is greater than the critical current I c for the GMR structure and is high enough to provide sufficient current density to cause the magnetization 211 of the free layer 210 to oscillate at a fixed base frequency in the absence of an external magnetic field.
- the sense current I s is less than the critical current I c for the TMR structure.
- a detector 350 is coupled to the circuitry for sense current I s .
- the detector 350 detects shifts in the free layer magnetization 211 oscillation frequency from the base frequency in response to the external magnetic fields from the magnetized regions of recording layer 254 .
- the current source 310 may instead apply an alternating current (AC) excitation current or an AC excitation current with a DC bias. This can allow for frequency locking of the oscillator to a fixed driving frequency, with associated pulling and detection of the magnetic field by phase detection, as is known in the literature, for example “Injection Locking and Pulling in Oscillators”, B Razavi, et al, IEEE J of Solid State Circuits 39, 1415 (2004), and U.S. Pat. No. 7,633,699).
- AC alternating current
- the single free layer 210 is a common free layer shared by the GMR and TMR structures and the three-terminal connection to the circuitry decouples I e from I s .
- the higher I e for exciting ST oscillations is passed through the conductive spacer 270 while the lower I s is passed through the tunnel barrier layer 230 to sense the oscillation of free layer magnetization 211 generated by I e through the GMR structure.
- the low I s keeps the voltage across the tunnel barrier layer 230 low to avoid dielectric breakdown, while still taking advantage of the much larger magnetoresistance signal of the TMR structure.
- the magnetizations 221 , 261 of the two reference layers 220 , 260 should be substantially parallel to one another to obtain the highest critical current.
- the magnetization 211 of free layer 210 should be substantially antiparallel to the magnetizations 221 , 261 of the two reference layers 220 , 260 , respectively, and substantially perpendicular to the ABS.
- the magnetization 211 of free layer 210 can point either toward or away from the recording layer 254 .
- the magnetizations 221 , 261 of the two reference layers 220 , 260 can be substantially antiparallel. This would lower the threshold current but can help to control the effective field on free layer 210 through magnetostatic interactions from the reference layers 220 , 260 .
- the manner of connection of the excitation current source 310 to the GMR structure defines the manner in which ST is imparted into the free layer 210 .
- the third terminal 303 is connected to conductive spacer layer 270 .
- This is the “reflection” mode because most electrons do not flow through the free layer 210 , but rather a spin current is generated by spin accumulation in the conductive spacer layer 270 that imparts ST to the free layer 210 .
- An alternative embodiment of the STO sensor 200 would be identical to FIG. 5 except that terminal 303 is connected to the free layer 210 instead of the conductive spacer layer 270 .
- This is the “transmission” mode because the electrons flow into the free layer 210 and directly impart ST to the free layer 210 .
- the transmission mode is more efficient in imparting ST and thus a smaller excitation current is required than is required for reflection mode.
- the density of the critical current I c may be on the order of 10 7 -10 8 A/cm 2 .
- An excitation current I e with a current density of 3-5 ⁇ 10 7 (transmission) or 1-5 ⁇ 10 8 (reflection) A/cm 2 would cause the magnetization 211 of free layer 210 to precess or oscillate at a resonance or base frequency of about 4-8 GHz (depending on the saturation magnetization of the ferromagnetic material used) in the absence of an external magnetic field.
- the positive and negative magnetizations in the recording layer 254 may generate magnetic fields of 100-500 Oe at the height at which the senor passes above the media and pass the free layer 210 at a frequency of up to 2 GHz. This field would cause shifts in the base frequency of oscillation of the magnetization 211 of free layer 210 of about ⁇ 1 GHz.
- the sense current I s would have a current density of about 10 7 A/cm 2 .
- the detector 350 can measure the frequency of oscillation of the free layer magnetization by measuring the change in electrical resistance across the tunnel barrier layer 230 .
- the frequency modulation (FM) signal from the free layer magnetization oscillations is converted to a train of voltage pulses (a digital signal) and a delay detection method is employed for the FM detection.
- the set of layers is in the form of a stack of layers with each layer deposited sequentially on the substrate, e.g., S 1 , with tunnel barrier layer 230 being in contact with one surface of free layer 210 and the conductive spacer layer 270 being in contact with the opposite surface of free layer 210 .
- FIG. 6 shows a modification of the embodiment of FIG.
- the second reference layer 260 a is not in the stack but is generally formed in the same plane as the conductive spacer layer 270 a .
- the second reference layer 260 a is shown recessed from the ABS in FIG. 6 but alternatively it could be located to either the side of the conductive spacer layer 270 a (the cross-track direction) and still be generally formed in the same plane as the conductive spacer layer 270 a . In either case the S 1 -S 2 shield-to-shield spacing is reduced from the embodiment of FIG. 5 .
- terminal 303 makes its connection to conductive spacer layer 270 a through second reference layer 260 a and the excitation current passes through the recessed second reference layer 260 a and the conductive spacer layer 270 a.
- FIG. 7 shows an embodiment with the STO sensor in transmission mode but wherein both the second reference layer 260 b and the nonmagnetic conductive spacer layer 270 b are not in the stack but are both generally formed in the same plane as the free layer 210 .
- the second reference layer 260 b and conductive spacer layer 270 b are shown recessed from the ABS in FIG. 7 but alternatively they could be located to either the side of the free layer 210 (the cross-track direction) and still be generally formed in the same plane as the free layer 210 . In either case the S 1 -S 2 shield-to-shield spacing is reduced from the embodiment of FIG. 5 .
- the excitation current passes through the recessed second reference layer 260 b , the conductive spacer layer 270 b and the free layer 210 .
- FIG. 8 shows an embodiment with the STO sensor in transmission mode but wherein both the first reference layer 220 a and the tunnel barrier layer 230 a are not in the stack but are both generally formed in the same plane as the free layer 210 .
- the first reference layer 220 a and tunnel barrier layer 230 a are shown recessed from the ABS in FIG. 8 but alternatively they could be located to either the side of the free layer 210 (the cross-track direction) and still be generally formed in the same plane as the free layer 210 . In either case the S 1 -S 2 shield-to-shield spacing is reduced from the embodiment of FIG. 5 .
- the excitation current passes from S 1 through the nonmagnetic spacer layer 270 and free layer 210 to S 2 .
- the properties of the materials used for the free layer in the CPP sensor can be chosen to reduce or increase I c , and thus change the level of excitation current I e that needs to be supplied. For example a lower I c may be desirable to limit the power dissipated in generating free layer oscillations,
- the use of certain types of materials for the free layer to change the excitation current in a STO sensor are described in application Ser. No. 12/188,183, filed Aug. 7, 2008 and assigned to the same assignee as this application.
- the critical current is given generally by the following:
- I C ( ⁇ / g ) M s t ( H k +2 ⁇ M s ),
- ⁇ is the damping parameter
- g is a parameter that depends on the spin-polarization of the ferromagnetic material
- M s is the saturation magnetization and t the thickness of the free layer
- H k is the anisotropy field of the free layer.
- the product M s *t is determined by the flux from the recorded bits on the disk and is typically given in terms of equivalent thicknesses of NiFe alloy, for example 40 ⁇ equivalent of permalloy ( ⁇ 800 emu/cm 3 ).
- a free layer material with desirable values for the parameters ⁇ , M s , and H k can be selected to change I c .
- Ni 81 Fe 19 exhibits a low a of about 0.01 to 0.02, low M s *t of about 800 emu/cm 3 and low intrinsic anisotropy field H k of about 1 Oe.
- the free ferromagnetic layer 210 may be formed of or comprise a ferromagnetic Heusler alloy, some of which are known to exhibit high spin-polarization in their bulk form.
- Full and half Heusler alloys are intermetallics with particular composition and crystal structure. Examples of Heusler alloys include but are not limited to the full Heusler alloys Co 2 MnX (where X is one or more of Al, Sb, Si, Sn, Ga, or Ge), Co 2 FeSi, and Co 2 Fe x Cr (1-x) Al (where x is between 0 and 1).
- Examples also include but are not limited to the half Heusler alloys NiMnSb, and PtMnSb.
- a perfect Heusler alloy will have 100% spin-polarization.
- the band structure of the Heusler alloy may deviate from its ideal half metal structure and that the spin polarization will decrease.
- some alloys may exhibit chemical site disorder and crystallize in the B 2 structure instead of the L 2 1 Heusler structure. Nevertheless, the spin polarization may exceed that of conventional ferromagnetic alloys.
- Heusler alloy shall mean an alloy with a composition substantially the same as that of a known Heusler alloy, and which results in enhanced spin polarization compared to conventional ferromagnetic materials such as NiFe and CoFe alloys.
- Another class of materials that can be used are those with short spin-diffusion length comparable to the thickness of a typical free layer. Similar to materials with high spin-polarization they are effective in scattering spins over a short length scale and thus induce spin-torque instabilities.
- One such preferred material has a composition of (Co x Fe 100-x ) (100-y) M y , where M is an element selected from the group consisting of Al, Ge and Si and where x is between about 40and 60 and y is between about 20 and 40. These materials have the advantage of reasonably high spin-polarization and low magnetic damping, which is desirable in the sensor of this invention to reduce I C .
- the invention has other applications.
- Other applications of the three-terminal STO, all of which would benefit from being able to use the sense current through the tunnel barrier layer to detect the frequency or phase of the free layer oscillation include mixers, radio, cell phones and radar (including vehicle radar). See for example, “STO frequency vs. magnetic field angle: The prospect of operation beyond 65 GHz”, by Bonetti et al, APL 94 102507 (2009).
- Still another application is for high-frequency assisted writing in magnetic recording, such as a magnetic recording disk drive.
- the STO applies a high-frequency oscillatory magnetic field to the magnetic grains of the recording layer as a magnetic field auxiliary to the magnetic write field from the conventional write head.
- the auxiliary field may have a frequency close to the resonance frequency of the magnetic grains in the recording layer to facilitate the switching of the magnetization of the grains at lower write fields from the conventional write head than would otherwise be possible without assisted recording.
- a two-terminal STO based on either GMR or TMR operates with the magnetization of the reference layer and the magnetization of the free layer, in the absence of an excitation current, oriented perpendicular to the planes of the layers. See for example “Microwave Assisted Magnetic Recording”, by J. G. Zhu et al., IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008, pp. 125-131.
- the three-terminal STO according to the invention like that shown in FIG.
- the magnetizations 221 , 261 of reference layers 220 , 260 , respectively, would be oriented perpendicular to the planes of the layers, and the magnetization 211 of the free layer 210 , in the absence of excitation current I e , would also be oriented perpendicular to the plane of the layer.
- the sense current I s through the tunnel barrier layer 230 is then used to monitor the frequency of the oscillation of the free layer magnetization 211 .
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Abstract
Description
- 1. Field of the Invention
- The invention relates generally to a spin-torque oscillator (STO), and more particularly to a magnetic field sensor and sensing system that uses a STO sensor.
- 2. Background of the Invention
- One type of conventional magnetoresistive (MR) sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field, such as from the recorded magnetic bits on the disk, is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
- In addition to CPP-GMR read heads, another type of CPP sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO or Al2O3.
- In CPP MR sensors, it is desirable to operate the sensors at a high bias or sense current density to maximize the signal and signal-to-noise ratio (SNR). However, it is known that CPP MR sensors are susceptible to current-induced noise and instability. The spin-polarized bias current flows perpendicularly through the ferromagnetic layers and produces a spin-torque (ST) effect on the local magnetization. This can produce fluctuations of the magnetization, resulting in substantial low-frequency magnetic noise if the sense current is large.
- An alternative sensor based on a CPP-GMR or CPP-TMR sensor structure, called a spin-torque oscillator (STO) sensor, is designed so that the spin torque effect generates persistent precession of the magnetization. When a fixed direct current higher than Ic, called the critical current, is directed through the STO sensor, the magnetization of the free layer precesses or oscillates by virtue of the ST effect. In appropriately designed structures the frequency of this precession (oscillation frequency) shifts with the application of an external magnetic field, and these frequency shifts can be used to detect changes in the external magnetic field. Thus, STO sensors have been proposed for use as read heads in magnetic recording disk drives to replace conventional CPP-GMR and CPP-TMR read heads, as described for example in US 20100328799 A1 assigned to the same assignee as this application, and in US 20090201614 A1.
- An STO sensor based on a CPP-GMR sensor can operate at very high current densities due to its nonmagnetic conductive spacer layer between the reference and free layers, but has a very small output signal as a result of its low magnetoresistance (AR/R). An STO sensor based on a CPP-TMR sensor has a significantly higher magnetoresistance but is susceptible to dielectric breakdown of the tunnel barrier at high current density.
- What is needed is a STO for use in a system, like a magnetic field sensing system, that has a high output signal that is not susceptible to dielectric breakdown of the tunnel barrier.
- The invention is a spin-torque oscillator (STO) with a single free layer that forms part of both a GMR structure with a nonmagnetic conductive spacer layer and a TMR structure with a tunnel barrier layer. The STO has three electrical terminals that connect to electrical circuitry that provides a spin-torque excitation current through the conductive spacer layer and a lesser sense current through the tunnel barrier layer. The STO has applications for use as an oscillator in mixers, radios, cell phones and radar (including vehicle radar), and as an oscillator in microwave-assisted magnetic recording (MAMR).
- In one specific application the STO is a magnetic field sensor, such as a current-perpendicular-to-the-plane (CPP) disk drive read head. In this application the STO sensor has a single free ferromagnetic layer that has an in-plane magnetization substantially free to oscillate in the presence of external magnetic fields to be sensed, such as the magnetized “bits” or regions on the disk. The free layer forms part of both a TMR structure with tunnel barrier layer and a first reference layer having a fixed in-plane magnetization, and a GMR structure with a nonmagnetic conductive spacer layer and a second reference layer having a fixed in-plane magnetization. The STO sensor has three electrical contacts or terminals for connection to electrical circuitry. A first terminal is electrically coupled to the first reference layer, a second terminal is electrically coupled to the second reference layer, and a third terminal is electrically coupled to either the conductive spacer layer or the free layer. The electrical circuitry connected to the STO terminals includes an excitation current source and a sense current source. The excitation current is greater that the critical current for the GMR structure and is high enough to provide sufficient current density to cause the magnetization of the free layer to oscillate at a fixed base frequency in the absence of an external magnetic field. The sense current is less than the critical current for the TMR structure. A detector coupled to the sense current detects shifts in the free layer magnetization oscillation frequency from the base frequency in response to the external magnetic fields from the magnetized regions of the disk.
- For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.
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FIG. 1 is a schematic top view of a conventional magnetic recording hard disk drive with the cover removed. -
FIG. 2 is an enlarged end view of the slider and a section of the disk taken in the direction 2-2 inFIG. 1 . -
FIG. 3 is a view in the direction 3-3 ofFIG. 2 and shows the air-bearing surface (ABS) of the slider with the ends of the read/write head. -
FIG. 4 is a cross-sectional schematic view of the ABS of a current-perpendicular-to-the-plane magnetoresistive (CPP MR) read head showing the stack of layers located between the magnetic shield layers according to the prior art. -
FIG. 5 is a schematic of a magnetic field spin-torque oscillator (STO) sensing system according to an embodiment of the invention in a magnetic recording disk drive implementation. -
FIG. 6 is a schematic of the embodiment ofFIG. 5 with the STO sensor in reflection mode but wherein the second reference layer is not in the stack but recessed from the ABS. -
FIG. 7 is a schematic of an embodiment with the STO sensor in transmission mode but wherein both the second reference layer and the nonmagnetic conductive spacer layer are not in the stack but recessed from the ABS. -
FIG. 8 is a schematic of an embodiment with the STO sensor in transmission mode but wherein both the first reference layer and the tunnel barrier layer are not in the stack but are recessed from the ABS. - The three-terminal STO according to the invention has applications other than as a magnetic field sensor, but will be described in detail below as magnetic recording disk drive read head.
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FIGS. 1-4 illustrate a conventional CPP magnetoresistive (MR) magnetic field sensing sensor and system.FIG. 1 is a block diagram of a conventional magnetic recording hard disk drive. The disk drive includes amagnetic recording disk 12 and a rotary voice coil motor (VCM)actuator 14 supported on a disk drive housing orbase 16. Thedisk 12 has a center ofrotation 13 and is rotated indirection 15 by a spindle motor (not shown) mounted tobase 16. Theactuator 14 pivots aboutaxis 17 and includes arigid actuator arm 18. A generallyflexible suspension 20 includes aflexure element 23 and is attached to the end ofarm 18. A head carrier or air-bearingslider 22 is attached to theflexure 23. A magnetic recording read/writehead 24 is formed on thetrailing surface 25 ofslider 22. Theflexure 23 andsuspension 20 enable the slider to “pitch” and “roll” on an air-bearing generated by the rotatingdisk 12. Typically, there are multiple disks stacked on a hub that is rotated by the spindle motor, with a separate slider and read/write head associated with each disk surface. -
FIG. 2 is an enlarged end view of theslider 22 and a section of thedisk 12 taken in the direction 2-2 inFIG. 1 . Theslider 22 is attached toflexure 23 and has an air-bearing surface (ABS) 27 facing thedisk 12 and atrailing surface 25 generally perpendicular to the ABS. TheABS 27 causes the airflow from the rotatingdisk 12 to generate a bearing of air that supports theslider 20 in very close proximity to or near contact with the surface ofdisk 12. The read/write head 24 is formed on the trailingsurface 25 and is connected to the disk drive read/write electronics by electrical connection toterminal pads 29 on the trailingsurface 25. As shown in the sectional view ofFIG. 2 , thedisk 12 is a patterned-media disk with discrete data tracks 50 spaced-apart in the cross-track direction, one of which is shown as being aligned with read/write head 24. The discrete data tracks 50 have a track width TW in the cross-track direction and may be formed of continuous magnetizable material in the circumferential direction, in which case the patterned-media disk 12 is referred to as a discrete-track-media (DTM) disk. Alternatively, the data tracks 50 may contain discrete data islands spaced-apart along the tracks, in which case the patterned-media disk 12 is referred to as a bit-patterned-media (BPM) disk. Thedisk 12 may also be a conventional continuous-media (CM) disk wherein the recording layer is not patterned, but is a continuous layer of recording material. In a CM disk the concentric data tracks with track width TW are created when the write head writes on the continuous recording layer. -
FIG. 3 is a view in the direction 3-3 ofFIG. 2 and shows the ends of read/write head 24 as viewed from thedisk 12. The read/write head 24 is a series of thin films deposited and lithographically patterned on the trailingsurface 25 ofslider 22. The write head includes a perpendicular magnetic write pole (WP) and may also include trailing and/or side shields (not shown). The CPP MR sensor or readhead 100 is located between two magnetic shields S1 and S2. The shields S1, S2 are formed of magnetically permeable material, typically a NiFe alloy, and may also be electrically conductive so they can function as the electrical leads to the readhead 100. The shields function to shield the readhead 100 from recorded data bits that are neighboring the data bit being read. Separate electrical leads may also be used, in which case theread head 100 is formed in contact with layers of electrically conducting lead material, such as tantalum, gold, or copper, that are in contact with the shields S1, S2.FIG. 3 is not to scale because of the difficulty in showing very small dimensions. Typically each shield S1, S2 is several microns thick in the along-the-track direction, as compared to the total thickness of the readhead 100 in the along-the-track direction, which may be in the range of 20 to 40 nm. -
FIG. 4 is a view of the ABS showing the layers making up a CPP MR sensor structure as would be viewed from the disk.Sensor 100 is a CPP MR read head comprising a stack of layers formed between the two magnetic shield layers S1, S2. Thesensor 100 has a front edge at the ABS and spaced-apart side edges 102, 104 that define the track width (TW). The shields S1, S2 are formed of electrically conductive material and thus may also function as electrical leads for the bias or sense current Is, which is directed generally perpendicularly through the layers in the sensor stack. Alternatively, separate electrical lead layers may be formed between the shields S1, S2 and the sensor stack. The lower shield S1 is typically polished by chemical-mechanical polishing (CMP) to provide a smooth substrate for the growth of the sensor stack. Aseed layer 101, such as a thin Ru/NiFe bilayer, is deposited, typically by sputtering, below S2 to facilitate the electroplating of the relatively thick S2. - The
sensor 100 layers include a referenceferromagnetic layer 120 having a fixed magnetic moment ormagnetization direction 121 oriented transversely (into the page), a freeferromagnetic layer 110 having a magnetic moment ormagnetization direction 111 that can rotate in the plane oflayer 110 in response to transverse external magnetic fields from thedisk 12, and anonmagnetic spacer layer 130 between thereference layer 120 andfree layer 110. TheCPP MR sensor 100 may be a CPP GMR sensor, in which case thenonmagnetic spacer layer 130 would be formed of an electrically conducting material, typically a metal like Cu, Au or Ag. Alternatively, theCPP MR sensor 100 may be a CPP tunneling MR (CPP-TMR) sensor, in which case thenonmagnetic spacer layer 130 would be a tunnel barrier formed of an electrically insulating material, like TiO2, MgO or Al2O3. - The pinned ferromagnetic layer in a CPP MR sensor may be a single pinned layer or an antiparallel (AP) pinned structure like that shown in
FIG. 4 . An AP-pinned structure has first (AP1) 122 and second (AP2) 120 ferromagnetic layers separated by a nonmagnetic antiparallel coupling (APC)layer 123 with the magnetization directions of the two AP-pinned ferromagnetic layers oriented substantially antiparallel. TheAP2 layer 120, which is in contact with thenonmagnetic APC layer 120 on one side and the sensor's electricallynonmagnetic spacer layer 130 on the other side, is typically referred to as the reference layer. TheAP1 layer 122, which is typically in contact with anantiferromagnetic layer 124 or hard magnet pinning layer on one side and thenonmagnetic APC layer 123 on the other side, is typically referred to as the pinned layer. The AP-pinned structure minimizes the net magnetostatic coupling between the reference/pinned layers and the CPP MR free ferromagnetic layer. The AP-pinned structure is sometimes also called a “laminated” pinned layer or a synthetic antiferromagnet (SAF). - The pinned layer in the CPP GMR sensor in
FIG. 4 is a well-known AP-pinned structure with reference ferromagnetic layer 120 (AP2) and a lower ferromagnetic layer 122 (AP1) that are antiferromagnetically coupled across an AP coupling (APC)layer 123. TheAPC layer 123 is typically Ru, Ir, Rh, Cr or alloys thereof. The AP1 and AP2 layers, as well as the freeferromagnetic layer 110, are typically formed of crystalline CoFe or NiFe alloys, or a multilayer of these materials, such as a CoFe/NiFe bilayer. The AP1 and AP2 ferromagnetic layers have theirrespective magnetization directions AP1 layer 122 may have its magnetization direction pinned by being exchange-coupled to an antiferromagnetic (AF)layer 124 as shown inFIG. 4 . TheAF layer 124 is typically a Mn alloy, e.g., PtMn, NiMn, FeMn, IrMn, PdMn, PtPdMn or RhMn. Alternatively, the AP-pinned structure may be “self-pinned”. In a “self pinned” sensor the AP1 and AP2layer magnetization directions free layer 110 is minimized and the effective pinning field of theAF layer 124, which is approximately inversely proportional to the net moment of the AP-pinned structure, remains high. - A
seed layer 125 may be located between the lower shield layer S1 and the AP-pinned structure. IfAF layer 124 is used, theseed layer 125 enhances the growth of theAF layer 124. Theseed layer 125 is typically one or more layers of NiFeCr, NiFe, Ta, Cu or Ru. Acapping layer 112 is located between the freeferromagnetic layer 110 and the upper shield layer S2. Thecapping layer 112 provides corrosion protection and may be a single layer or multiple layers of different materials, such as Ru, Ta, Ti, or a Ru/Ta/Ru , Ru/Ti/Ru, or Cu/Ru/Ta trilayer. - In the presence of an external magnetic field in the range of interest, i.e., magnetic fields from recorded data on the disk, the
magnetization direction 111 offree layer 110 will rotate while themagnetization direction 121 ofreference layer 120 will remain fixed and not rotate. Thus when a sense current IS is applied from top shield S2 perpendicularly through the sensor stack to bottom shield S1 (or from S1 to S2), the magnetic fields from the recorded data on the disk will cause rotation of the free-layer magnetization 111 relative to the reference-layer magnetization 121, which is detectable as a change in electrical resistance. - A
ferromagnetic biasing layer 115, such as a CoPt or CoCrPt hard magnetic bias layer, is also typically formed outside of the sensor stack near the side edges 102, 104 of thesensor 100. Thebiasing layer 115 is electrically insulated fromside edges 102, 104 ofsensor 100 by insulatinglayer 116. Anoptional seed layer 114, such as a Cr alloy like CrMo or CrTi, may be deposited on the insulatinglayer 116 to facilitate the growth of thebiasing layer 115, particularly if the biasing layer is a CoPt or CoPtCr layer. Acapping layer 118, such as layer of Cr, or a multilayer of Ta/Cr is deposited on top of thebiasing layer 115. The upper layer of cappinglayer 118, for example Cr, also serves the purpose as a chemical-mechanical-polishing (CMP) stop layer during fabrication of the sensor. Thebiasing layer 115 has amagnetization 117 generally parallel to the ABS and thus longitudinally biases themagnetization 111 of thefree layer 110. Thus in the absence of an external magnetic field itsmagnetization 117 is parallel to themagnetization 111 of thefree layer 110. Theferromagnetic biasing layer 115 may be a hard magnetic bias layer or a ferromagnetic layer that is exchange-coupled to an antiferromagnetic layer. Aseed layer 101, such as a NiFe layer, for the shield layer S2 may be located over thesensor 100 andcapping layer 118. - In the presence of an external magnetic field in the range of interest, i.e., magnetic fields from recorded data on the
disk 12, themagnetization direction 111 offree layer 110 will rotate while themagnetization direction 121 ofreference layer 120 will remain substantially fixed and not rotate. The rotation of the free-layer magnetization 111 relative to the reference-layer magnetization 121 results in a change in electrical resistance. Hence, when a sense direct current IS is directed through the stack of layers insensor 100, the resistance change is detected as a voltage signal proportional to the strength of the magnetic signal fields from the recorded data on the disk. If Is is greater than some critical current (Ic) the spin-torque (ST) effect can produce gyrations or fluctuations of the free layer magnetization, resulting in substantial low-frequency magnetic noise that reduces the sensor's signal-to-noise ratio (SNR) to an undesirable level. - An alternative sensor based on a CPP-GMR or CPP-TMR sensor, called a spin torque oscillator (STO) sensor, operates at a sense current greater than L to take advantage of the ST-induced forces acting on the free layer. When a fixed direct current higher than Ic is directed through this type of STO sensor, the magnetization of the free layer precesses or oscillates by virtue of the ST effect. The frequency of this precession (oscillation frequency) shifts with the application of an external magnetic field, and these frequency shifts can be used to detect changes in the external magnetic field. Thus, STO sensors have been proposed for use as read heads in magnetic recording disk drives to replace conventional CPP-GMR and CPP-TMR read heads, as described for example in US 20100328799 A1 assigned to the same assignee as this application, and in US 20090201614 A1.
- An STO sensor based on a CPP-GMR sensor can operate at very high current densities due to its conductive spacer layer between the reference and free layers, but has a very small output signal as a result of its low magnetoresistance (ΔR). An STO sensor based on a CPP-TMR sensor has a significantly higher magnetoresistance but is susceptible to dielectric breakdown of the tunnel barrier at high current density.
- The STO sensor according to the present invention uses the positive aspects of both CPP-GMR and CPP-TMR sensors to operate at a high current density and to provide a high output signal.
FIG. 5 is a schematic of a magnetic field sensing system using aSTO sensor 200 according to an embodiment of the invention. The system is illustrated as a magnetic recording disk drive withSTO sensor 200 with its ABS facing thedisk 250. Thesensor 200 includes a set of individual layers and features of both a CPP-GMR sensor and a CPP-TMR sensor as previously-described with respect toCPP sensor 100. Thedisk 250 has asubstrate 252 and arecording layer 254 that serves as the magnetic recording medium with magnetized regions depicted by the arrows directed toward or away from the ABS. As the disk rotates, the magnetized regions move in the direction ofarrow 215 past thesensor 200. Therecording layer 254 is depicted as a perpendicular magnetic recording medium with the regions magnetized perpendicularly to the plane ofrecording layer 254, but alternatively it may be a longitudinal magnetic recording medium with the regions being magnetized in the plane ofrecording layer 254. TheSTO sensor 200 has a first shield layer S1 that may serve as a substrate for the deposition of the set of layers, a second shield layer S2, and a single freeferromagnetic layer 210 that has a substantially in-plane magnetization 211 free to oscillate in the presence of an external magnetic field to be sensed. Thefree layer 210 forms part of both a TMR structure withtunnel barrier layer 230 and afirst reference layer 220 having a fixed in-plane magnetization 221, and a GMR structure with nonmagneticconductive spacer layer 270 andsecond reference layer 260 having a fixed in-plane magnetization 261. Each of the reference layers 220, 260 may be a single pinned layer or the AP2 layer of an AP-pinned structure. A non-ferromagneticconducting metal layer 225 is located between S1 andfirst reference layer 220 for breaking any magnetic exchange interaction between S1 andfirst reference layer 220 or other ferromagnetic layers in the sensor stack while permitting electrical conduction. Similarly, a non-ferromagneticconducting metal layer 265 is located between S2 andsecond reference layer 260. Typical materials forlayers FIG. 5 , withsecond reference layer 260 being deposited first onlayer 265 on S1, followed byconductive spacer layer 270,free layer 210,tunnel barrier layer 230,first reference layer 220, andlayer 225, with S2 being located onlayer 225. - The
STO sensor 200 has three electrical contacts or terminals for connection to electrical circuitry 300.Terminal 301 is electrically coupled to thefirst reference layer 220 via S1, terminal 302 is electrically coupled to thesecond reference layer 260 via S2, andterminal 303 is electrically coupled to either theconductive spacer layer 270 or to thefree layer 210. In the embodiment ofFIG. 5 terminal 303 is connected toconductive spacer layer 270, but may be connected to either theconductive spacer layer 270 or thefree layer 210. Theterminals FIG. 5 as being directly connected to their respective layers, but would likely be located on the trailing surface of the slider, as depicted inFIG. 2 byterminal pads 29 on trailingsurface 25 ofslider 22. The sensor includes insulatingmaterial 290 in the back region recessed from the ABS for electrically isolating S1,conductive spacer layer 270 and S2 from one another. - The circuitry connected to
STO sensor 200 includes a constantcurrent source 310 that supplies a direct current (DC) excitation current Ie betweenterminals conductive spacer layer 270, and a constantcurrent source 320 that supplies a direct DC sense current Is betweenterminals tunnel barrier layer 230. The excitation current is greater than the critical current Ic for the GMR structure and is high enough to provide sufficient current density to cause themagnetization 211 of thefree layer 210 to oscillate at a fixed base frequency in the absence of an external magnetic field. The sense current Is is less than the critical current Ic for the TMR structure. Adetector 350 is coupled to the circuitry for sense current Is. Thedetector 350 detects shifts in thefree layer magnetization 211 oscillation frequency from the base frequency in response to the external magnetic fields from the magnetized regions ofrecording layer 254. Thecurrent source 310 may instead apply an alternating current (AC) excitation current or an AC excitation current with a DC bias. This can allow for frequency locking of the oscillator to a fixed driving frequency, with associated pulling and detection of the magnetic field by phase detection, as is known in the literature, for example “Injection Locking and Pulling in Oscillators”, B Razavi, et al, IEEE J of Solid State Circuits 39, 1415 (2004), and U.S. Pat. No. 7,633,699). - The single
free layer 210 is a common free layer shared by the GMR and TMR structures and the three-terminal connection to the circuitry decouples Ie from Is. The higher Ie for exciting ST oscillations is passed through theconductive spacer 270 while the lower Is is passed through thetunnel barrier layer 230 to sense the oscillation offree layer magnetization 211 generated by Ie through the GMR structure. The low Is keeps the voltage across thetunnel barrier layer 230 low to avoid dielectric breakdown, while still taking advantage of the much larger magnetoresistance signal of the TMR structure. - In the preferred embodiment the
magnetizations reference layers magnetization 211 offree layer 210 should be substantially antiparallel to themagnetizations reference layers magnetization 211 offree layer 210 can point either toward or away from therecording layer 254. In an alternative embodiment, themagnetizations reference layers free layer 210 through magnetostatic interactions from the reference layers 220, 260. - The manner of connection of the excitation
current source 310 to the GMR structure defines the manner in which ST is imparted into thefree layer 210. In the embodiment ofFIG. 5 , thethird terminal 303 is connected toconductive spacer layer 270. This is the “reflection” mode because most electrons do not flow through thefree layer 210, but rather a spin current is generated by spin accumulation in theconductive spacer layer 270 that imparts ST to thefree layer 210. An alternative embodiment of theSTO sensor 200 would be identical toFIG. 5 except that terminal 303 is connected to thefree layer 210 instead of theconductive spacer layer 270. This is the “transmission” mode because the electrons flow into thefree layer 210 and directly impart ST to thefree layer 210. The transmission mode is more efficient in imparting ST and thus a smaller excitation current is required than is required for reflection mode. - As one example of a disk drive STO sensor according to the invention operating in reflection mode, the density of the critical current Ic may be on the order of 107-108 A/cm2. An excitation current Ie with a current density of 3-5×107(transmission) or 1-5×108 (reflection) A/cm2 would cause the
magnetization 211 offree layer 210 to precess or oscillate at a resonance or base frequency of about 4-8 GHz (depending on the saturation magnetization of the ferromagnetic material used) in the absence of an external magnetic field. The positive and negative magnetizations in therecording layer 254 may generate magnetic fields of 100-500 Oe at the height at which the senor passes above the media and pass thefree layer 210 at a frequency of up to 2 GHz. This field would cause shifts in the base frequency of oscillation of themagnetization 211 offree layer 210 of about ±1 GHz. The sense current Is would have a current density of about 107 A/cm2. Thedetector 350 can measure the frequency of oscillation of the free layer magnetization by measuring the change in electrical resistance across thetunnel barrier layer 230. In one detection technique, the frequency modulation (FM) signal from the free layer magnetization oscillations is converted to a train of voltage pulses (a digital signal) and a delay detection method is employed for the FM detection. (K. Mizushima, et al., “Signal-to-noise ratios in high-signal-transfer-rate read heads composed of spin-torque oscillators”, J. Appl. Phys. 107, 063904 2010). - For magnetic recording applications it is desirable to fit the STO sensor layers into as narrow a space as possible between the magnetic shields to achieve the highest spatial resolution of the recorded magnetic bits in the along-the-track direction (parallel to the direction of
arrow 215 inFIG. 5 ). InFIG. 5 , the set of layers is in the form of a stack of layers with each layer deposited sequentially on the substrate, e.g., S1, withtunnel barrier layer 230 being in contact with one surface offree layer 210 and theconductive spacer layer 270 being in contact with the opposite surface offree layer 210.FIG. 6 shows a modification of the embodiment ofFIG. 5 with the STO sensor in reflection mode but wherein thesecond reference layer 260 a is not in the stack but is generally formed in the same plane as the conductive spacer layer 270 a. Thesecond reference layer 260 a is shown recessed from the ABS inFIG. 6 but alternatively it could be located to either the side of the conductive spacer layer 270 a (the cross-track direction) and still be generally formed in the same plane as the conductive spacer layer 270 a. In either case the S1-S2 shield-to-shield spacing is reduced from the embodiment ofFIG. 5 . In this reflection-mode embodiment terminal 303 makes its connection to conductive spacer layer 270 a throughsecond reference layer 260 a and the excitation current passes through the recessedsecond reference layer 260 a and the conductive spacer layer 270 a. -
FIG. 7 shows an embodiment with the STO sensor in transmission mode but wherein both thesecond reference layer 260 b and the nonmagneticconductive spacer layer 270 b are not in the stack but are both generally formed in the same plane as thefree layer 210. Thesecond reference layer 260 b andconductive spacer layer 270 b are shown recessed from the ABS inFIG. 7 but alternatively they could be located to either the side of the free layer 210 (the cross-track direction) and still be generally formed in the same plane as thefree layer 210. In either case the S1-S2 shield-to-shield spacing is reduced from the embodiment ofFIG. 5 . In this transmission-mode embodiment the excitation current passes through the recessedsecond reference layer 260 b, theconductive spacer layer 270 b and thefree layer 210. -
FIG. 8 shows an embodiment with the STO sensor in transmission mode but wherein both thefirst reference layer 220 a and thetunnel barrier layer 230 a are not in the stack but are both generally formed in the same plane as thefree layer 210. Thefirst reference layer 220 a andtunnel barrier layer 230 a are shown recessed from the ABS inFIG. 8 but alternatively they could be located to either the side of the free layer 210 (the cross-track direction) and still be generally formed in the same plane as thefree layer 210. In either case the S1-S2 shield-to-shield spacing is reduced from the embodiment ofFIG. 5 . In this transmission-mode embodiment the excitation current passes from S1 through thenonmagnetic spacer layer 270 andfree layer 210 to S2. - Because in the present invention it is necessary that the
STO sensor 200 operates at current levels above Ic to induce the spin-torque effect in thefree layer 210, the properties of the materials used for the free layer in the CPP sensor can be chosen to reduce or increase Ic, and thus change the level of excitation current Ie that needs to be supplied. For example a lower Ic may be desirable to limit the power dissipated in generating free layer oscillations, The use of certain types of materials for the free layer to change the excitation current in a STO sensor are described in application Ser. No. 12/188,183, filed Aug. 7, 2008 and assigned to the same assignee as this application. - The critical current is given generally by the following:
-
I C=(α/g)M s t(H k+2πM s), - where α is the damping parameter, g is a parameter that depends on the spin-polarization of the ferromagnetic material, Ms is the saturation magnetization and t the thickness of the free layer, and Hk is the anisotropy field of the free layer. The product Ms*t is determined by the flux from the recorded bits on the disk and is typically given in terms of equivalent thicknesses of NiFe alloy, for example 40 Å equivalent of permalloy (˜800 emu/cm3). Thus a free layer material with desirable values for the parameters α, Ms, and Hk can be selected to change Ic. For example, Ni81Fe19 exhibits a low a of about 0.01 to 0.02, low Ms*t of about 800 emu/cm3 and low intrinsic anisotropy field Hk of about 1 Oe.
- Also, high spin-polarization materials will decrease IC significantly by increasing the value of the parameter g, which depends on the spin-polarization of the ferromagnetic material. Thus the free
ferromagnetic layer 210 may be formed of or comprise a ferromagnetic Heusler alloy, some of which are known to exhibit high spin-polarization in their bulk form. Full and half Heusler alloys are intermetallics with particular composition and crystal structure. Examples of Heusler alloys include but are not limited to the full Heusler alloys Co2MnX (where X is one or more of Al, Sb, Si, Sn, Ga, or Ge), Co2FeSi, and Co2FexCr(1-x)Al (where x is between 0 and 1). Examples also include but are not limited to the half Heusler alloys NiMnSb, and PtMnSb. A perfect Heusler alloy will have 100% spin-polarization. However it is possible that in a thin-film form and at finite temperatures, the band structure of the Heusler alloy may deviate from its ideal half metal structure and that the spin polarization will decrease. For example, some alloys may exhibit chemical site disorder and crystallize in the B2 structure instead of the L2 1 Heusler structure. Nevertheless, the spin polarization may exceed that of conventional ferromagnetic alloys. Thus, as used herein a “Heusler alloy” shall mean an alloy with a composition substantially the same as that of a known Heusler alloy, and which results in enhanced spin polarization compared to conventional ferromagnetic materials such as NiFe and CoFe alloys. - Another class of materials that can be used are those with short spin-diffusion length comparable to the thickness of a typical free layer. Similar to materials with high spin-polarization they are effective in scattering spins over a short length scale and thus induce spin-torque instabilities. One such preferred material has a composition of (CoxFe100-x)(100-y)My, where M is an element selected from the group consisting of Al, Ge and Si and where x is between about 40and 60 and y is between about 20 and 40. These materials have the advantage of reasonably high spin-polarization and low magnetic damping, which is desirable in the sensor of this invention to reduce IC.
- As previously mentioned, while the three-terminal STO according to the invention has been described in detail with application as a magnetic field sensor, in particular a magnetic recording disk drive read head, the invention has other applications. Other applications of the three-terminal STO, all of which would benefit from being able to use the sense current through the tunnel barrier layer to detect the frequency or phase of the free layer oscillation include mixers, radio, cell phones and radar (including vehicle radar). See for example, “STO frequency vs. magnetic field angle: The prospect of operation beyond 65 GHz”, by Bonetti et al, APL 94 102507 (2009).
- Still another application is for high-frequency assisted writing in magnetic recording, such as a magnetic recording disk drive. In this technique, also called microwave-assisted magnetic recording (MAMR), the STO applies a high-frequency oscillatory magnetic field to the magnetic grains of the recording layer as a magnetic field auxiliary to the magnetic write field from the conventional write head. The auxiliary field may have a frequency close to the resonance frequency of the magnetic grains in the recording layer to facilitate the switching of the magnetization of the grains at lower write fields from the conventional write head than would otherwise be possible without assisted recording. In one type of MAMR system, a two-terminal STO based on either GMR or TMR, operates with the magnetization of the reference layer and the magnetization of the free layer, in the absence of an excitation current, oriented perpendicular to the planes of the layers. See for example “Microwave Assisted Magnetic Recording”, by J. G. Zhu et al., IEEE Transactions on Magnetics, Vol. 44, No. 1, January 2008, pp. 125-131. Thus when the three-terminal STO according to the invention, like that shown in
FIG. 5 , is used as a STO for MAMR, themagnetizations reference layers magnetization 211 of thefree layer 210, in the absence of excitation current Ie, would also be oriented perpendicular to the plane of the layer. The sense current Is through thetunnel barrier layer 230 is then used to monitor the frequency of the oscillation of thefree layer magnetization 211. - While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.
Claims (21)
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JP2012122079A JP2012253344A (en) | 2011-05-31 | 2012-05-29 | Three-terminal spin torque oscillator (sto) |
CN2012101771430A CN102810317A (en) | 2011-05-31 | 2012-05-31 | Spin-torque oscillator, sensor, magnetic field sensing system, and disk drive |
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JP2012253344A (en) | 2012-12-20 |
US8320080B1 (en) | 2012-11-27 |
CN102810317A (en) | 2012-12-05 |
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